Pyrimido(4,5-c)pyridazines

ABSTRACT

The present invention provides pyrimido(4,5-c)pyridazines, to methods for preparing them, formulations containing them and the preparation of such formulation and the use of such compounds in human therapy. These pyrimido(4,5-c)pyridazines of this invention are useful due to their activity as inhibitors of dihydropteroic and biosynthesis.

This invention relates to pyrimido(4,5-c)-pyridazines, their methods ofsynthesis, formulations containing them and their use as inhibitors ofdihydropteroic acid biosynthesis (DHPB).

The first pyramido(4,5-c)pyridazines were disclosed by Pfleiderer andFerch in 1958, Am. Chem., 615, 48 (1958) but no pharmacological activitywas disclosed for these compounds which have the formula (I): ##STR1##wherein R is a hydrogen atom or --CO₂ C₂ H₅ group. We have nowdiscovered a group of pyrimido(4,5-c)pyridazines which are useful asinhibitors of dihydropteroic acid biosynthesis (DHPB).

The present invention provides novel pyrimido (4,5-c)pyridazines offormula (II), or their tautomers, or salts thereof, ##STR2## wherein R¹is a lower alkyl group, a hydroxymethyl group, a phenyl group, a carboxygroup, a benzyl group optionally substituted in the phenyl ring with oneor more nitro or lower alkoxy groups, a phenacyl group optionallysubstituted in the phenyl ring with one or more hydroxy or lower alkoxygroups, a lower acyloxymethyl group, an indolyl or indolylmethyl group,a group CH(CN)CH₂ C₆ H₅ optionally substituted in the phenyl ring withone or more lower alkoxy groups, a group CH(Y)CO₂ Z or a group CH₂ CH₂CO₂ Z in which Y is a hydrogen atom or a lower alkyl or alkoxy group andZ is a hydrogen atom or a lower alkyl group.

The term "lower" as used herein in conjunction with an alkyl, alkoxy oracyl group is indicative of the fact that such groups have from 1 to 6carbon atoms arranged in a straight or branched chain. The expression"phenacyl group" however is used to denote solely a C₆ H₅ COCH₂ --group.

It is to be understood that compounds where tautomerism is possiblebetween, on the one hand, a hydroxy group and an oxo group, and on theother hand, an amino group and an imino group, at a particular positionin either of the rings of the pyrimido(4,5-c)-pyridazines of formula(II), the more stable forms are respectively, the oxo group and theamino group. However, the general formulae used in the presentspecification do not necessarily represent the more stable forms of suchpyridazines.

The above compounds of formula (II) inhibit the enzymedihydropteroatesynthetase which enables microorganisms to synthesise anessential intermediate in the production of tetrahydrofolate co-factors.Most of these co-factors are one-carbon adducts of tetrahydrofolic acidand they are essential metabolites in cells for the biosynthesis ofpurines, thymidylic acid, serine, and several other biologicallyimportant compounds. Man and other higher animals are unable tosynthesise such co-factors and therefore they have to obtain them fromfood which contains the required preformed folates.

On the other hand, microorganisms synthesise the co-factors themselvesfrom simpler chemicals. Generally the biosynthetic process firstprovides `dihydropteridine` (Pt), i.e.2-amino-4-hydroxy-6-hydroxymethyl-7,8-dihydropteridine (HMPt)pyrophosphate ester, from its immediate precursor HMPt in the presenceof the enzyme hydroxymethyldihydropteridine pyrophosphokinase (HMPPs).Pt then condenses with p-aminobenzoic acid (pAB) in the presence of theenzyme dihydropteroatesynthetase to form dihydropteroic acid (DPtA).This intermediate further condenses with a glutamate to formdihydrofolic acid (DFA or `folate`) which is then enzymatically reducedto produce the essential tetrahydrofolate. It is in the formation ofDPtA from pAB and Pt that the present compounds have inhibitoryactivity.

On the basis of such inhibitory activity the pyrimido(4,5-c)pyridazinesof formula (II) have anti-microbial, in particular anti-bacterial,activity.

Within the class of pyrimido(4,5-c)pyridazines of formula (II) there isa group of compounds which are particularly active and these have R¹ asa methyl group, a phenyl group, a benzyl group optionally substituted inthe phenyl ring with a nitro or 2 or 3 methoxy groups, a hydroxymethylgroup, a phenacyl group optionally substituted in the phenyl ring with ahydroxy group, or 2 or 3 methoxy groups, a group CH₂ CH₂ CO₂ H, anacetyloxymethyl group, an indolylmethyl group or a group CH(CN)CH₂ C₆ H₅substituted in the phenyl ring with 3 methoxy groups, a group CH(Y)CO₂ Zin which Y is a methyl group, methoxy group or a hydrogen atom and Z isa hydrogen atom, or a C₁₋₄ alkyl group.

As examples of compounds which are particularly active and which fallwithin this class are7-amino-1,3-dimethyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)-pyridazine;7-amino-1-methyl-3-phenyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-hydroxymethyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-benzyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-(2-nitrobenzyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-indolylmethyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-acetoxymethyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)-pyridazine;7-amino-1-methyl-3-(1-carboxyethyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-(2-carboxymethyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-(ethoxycarbonylmethyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido-(4,5-c)pyridazine;7-amino-1-methyl-3-((1-methoxy)-carboxymethyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido-(4,5-c)pyridazine;7-amino-1-methyl-3-(α-cyano-3,4,5-trimethoxyphenethyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;and especially7-amino-1-methyl-3-(1-ethoxycarbonylethyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine.

However, a more preferred class of even higher activity compounds, arethose of formula (II) wherein R¹ is a benzyl group or especially whereinR¹ is a phenacyl group optionally substituted in the phenyl ring with ahydroxy group or 2 or 3 methoxy groups. Examples of compounds fallingwithin this most preferred class are7-amino-3-benzoylmethyl-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-3-(3,4-dimethoxybenzoyl)methyl-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-3-(3,4,5-trimethoxybenzoyl)methyl-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-3-(3,4,5-trimethoxybenzoyl)methyl-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-3-(2,4-dimethoxybenzoyl)methyl-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-3-(3,4,5-trimethoxybenzyl)-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-3-(2,5-dimethoxybenzoyl)methyl-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-3-(2,4,6-trimethoxybenzoyl)methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-3-(3-hydroxybenzoyl)-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)-pyridazine;and7-amino-3-(3,4-dimethoxybenzyl)-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine.

It has previously been stated that in 1958 Pfleiderer W. and Ferch H.(Justus Liebig's Ann. Chem., 1958, 615, 48) reported the preparation of4-hydroxy-6,8-dimethylpyrimido(4,5-c)pyridazine-5,7-(6H, 8H)-dione bythe cyclisation of glyoxylic acid ethylester-1,3-dimethyluracil-(4)-hydrazone. It has now been found that thiscyclisation reaction can surprisingly be extended to a novel class ofintermediates which have a number of different substituents. Thus thepresent invention further provides a method of preparing a compound offormula (II), or a tautomer or a salt thereof, as hereinbefore definedwhich process comprises the cyclisation of a compound of the formula(III): ##STR3## wherein R³ is a lower alkyl group and R² is a loweralkyl group, a lower acyloxymethyl group, a phenyl group, a benzyl groupoptionally substituted in the phenyl ring with one or more nitro orlower alkoxy groups, a phenacyl group optionally substituted in thephenyl ring with one or more hydroxy or lower alkoxy groups, an indolylor indolylmethyl group, a group CH(CN)CH₂ C₆ H₅ optionally substitutedin the phenyl ring with one or more lower alkoxy groups, a groupCH(Y)CO₂ Z or a group CH₂ CH₂ CO₂ Z in which Y is a hydrogen atom or alower alkyl or alkoxy group and Z is a hydrogen atom or a lower alkylgroup and thereafter optionally hydrolysing the lower acyloxymethylgroup to a hydroxymethyl group.

Such is the nature of the substituents on the pyrimidine ring of thecompounds of formula (III) that, unlike the above prior art teaching,ring closure can apparently only be achieved when the nitrogen atom atthe 6-position is substituted as hereinabove. In other words, when thisparticular nitrogen atom is unsubstituted, corresponding compounds offormula (III) do not appear to cyclise. Moreover, this cyclisationreaction is particularly surprising since the report of Pfleiderer andFerch teaches that such reactions only work for those hydrazoneintermediates which have a glyoxylic acid alkyl ester substitution, yeta corresponding substitution in the present intermediates results inlittle, if any, pyrimido(4,5-c)pyridazine.

The reaction itself may be carried out in any suitable solvent but mostdesirably a hydroxylic solvent, for example glacial acetic acid, water,of C₁₋₄ alkanol, at reflux temperature for up to several days.Optimally, the reaction is carried out in refluxing methanol, or inethanol at the reflux temperature of methanol.

The hydrolysis of the lower acyloxymethyl group will preferably takeplace under alkaline conditions, for example, by using aqueous sodiumhydroxide. The hydrolysis will be carried out at a non extremetemperature, i.e. between 10° and 100° C., and preferably at roomtemperature.

The compounds of formula (III) are novel and constitute a further aspectof this invention.

The compounds of formula (III) can be prepared, preferably in situ, bycondensing a 2-amino-4-oxo-6-hydrazinopyrimidine of formula (IV) (or atautomer thereof). ##STR4## with an α-keto ester of formula (V):

    R.sup.2 CO.CO.OR.sup.3                                     (V)

wherein R² and R³ are as hereinbefore defined.

The preparation is suitably achieved using the conditions as specifiedfor the cyclisation reaction above, for example by refluxing thereactants in methanol.

The compound of the formula (IV) are novel and constitutes a furtheraspect of this invention.

In the preparation of those compounds of formula (II), in which R¹ is agroup CH(Y)CO₂ Z, a group CH₂ CH₂ CO₂ Z, or an optionally substitutedphenacyl group, some other bi-cyclic compound may be formed as aby-product. In such instances it may be necessary to isolate therequired compound by the usual procedures known in the art.

The compounds of formula (II) wherein R¹ is a group CH(Y)CO₂ Z or agroup CH₂ CH₂ CO₂ Z in which Y is as hereinbefore defined and Z is alower alkyl group may be hydrolysed to give further compounds of formula(II) wherein R¹ is a group CH(Y)CO₂ Z or a group CH₂ CH₂ CO₂ Z in whichY is as hereinbefore defined and Z is a hydrogen atom. The startingcompounds of formula (II) may be prepared from the correspondingcompound of formula (III) as described previously.

The conditions for this reaction are preferably alkaline which may beachieved by using, for instance, aqueous sodium hydroxide, and thereaction may be conveniently performed at room temperature for 15 to 150minutes, for example 90 minutes.

Compounds of formula (II) wherein R¹ is other than an acyloxymethylgroup or any group containing an ester function may be prepared by thehydrolysis of a compound of formula (VI): ##STR5## wherein R² is ashereinbefore defined.

The conditions for this reaction are preferably alkaline which may beachieved by using, for instance, aqueous sodium hydroxide, and thereaction may be conveniently performed under reflux for 10 to 40 hours,for example 24 hours. However, it should be noted that during the courseof this reaction some decarboxylation may take place, possibly givingrise to small amounts of by-products which may necessitate subsequentseparation by known methods.

This hydrolysis reaction is not preferred for those compounds of theformula (VI) wherein R² is a group which may undergo hydrolyticcleavage, for example when R² is an optionally substituted phenacylgroup if it is desired to prepare a compound of the formula (II) havingthe substituent R¹ corresponding to R².

The compounds of formula (VI) are novel and constitute a yet furtheraspect of the present invention.

The compounds of formula (VI), may be prepared by the cyclisation of acompound of formula (VII): ##STR6## wherein R² and R³ are ashereinbefore defined.

The reaction may be carried out in any suitable solvent but mostdesirably a hydroxylic solvent, for example glacial acetic acid, water,or C₁₋₄ alkanol, at reflux temperature for up to several days.Optimally, the reaction is carried out in refluxing methanol, or inethanol at the reflux temperature of methanol.

The compounds of formula (VII) are novel and constitute a further aspectof the present invention.

The compounds of formula (VII) can be prepared, preferably in situ, bycondensing a 2-amino-4-imino-6-hydrazinopyrimidine of formula (VIII), ora tautomer thereof, ##STR7## with an α-keto ester of formula (V).

The compound of formula (VIII) its tautomers and salts thereof are noveland provide a further aspect of the invention.

The preparation is suitably achieved using the conditions as specifiedfor the cyclisation reaction immediately above, for example by refluxingthe reactants in methanol.

In the preparation of those compounds of formula (VI) in which R² is agroup CH(Y)CO₂ Z, a group CH₂ CH₂ CO₂ Z, or an optionally substitutedphenacyl group, some other bicyclic compound may be formed as aby-product. In such instances it may be necessary to isolate therequired compound by the usual procedures known in the art.

It should be noted that although, in general, hydrolysis of a compoundof formula (VI) results in a correspondingly substituted compound offormula (II) except that the 5-position is oxo rather than iminosubstituted; in the case wherein R² in the starting material is a groupCH(Y)CO₂ Z or a group CH₂ CH₂ CO₂ Z in which Z is a lower alkyl group, Zin the end-product of formula (II) is a hydrogen atom.

All the starting materials specified above for the various syntheses maybe prepared by standard methods taught in the art.

The compounds of formula (II), or their tautomers, or pharmaceuticallyacceptable salts thereof may be presented in association with a carrierin pharmaceutical formulations suitable for parenteral, topical, rectalor oral administration. The formulations for oral or rectaladministration are advantageously presented in discrete units, such astablets, capsules, cachets, ampoules or suppositories, each containing apredetermined amount of compound, but may also be presented as a powder,as granules, as a solution or suspension in an aqueous or non-aqueousliquid, or as an ointment or paste for topical administration. Forparenteral use, the formulations incorporating an aqueous or non-aqueousliquid carrier must be sterile and be presented in sealed containers.The formulations may be made by any of the known methods and may includeone or more of the following accessory ingredients: diluents, solutes torender the solution isotonic with the blood, buffers, flavouring,binding, dispersing, surface-active, thickening, lubricating and coatingmaterials, preservatives, bacteriostats, antioxidants, suppository andointment bases, and any other acceptable excipients.

In another aspect of the present invention, therefore, there is provideda pharmaceutical formulation comprising a compound of formula (II) incombination with a pharmaceutically acceptable carrier. In yet anotheraspect the present invention provides a method of making apharmaceutical formulation by admixing the compound of formula (II) witha carrier by known techniques.

The compounds of formula (II), for use alone, may be presented in theform of their pharmaceutically acceptable salts. Examples ofpharmaceutically acceptable salts are those derived from mineral ororganic acids, for example hydrochloric acid, hydrobromic acid,sulphuric acid, acetic acid, citric acid, tartaric acid, lactic acid,maleic acid, or salicylic acid. Acid addition salts which are notpharmaceutically acceptable may be rendered so by a conventionalmetathetical reaction. Further examples of pharmaceutically acceptablesalts are, in the case when R¹ in formula (II) is a carboxy group, agroup CH(Y)CO₂ Z, or a group CH₂ CH₂ CO₂ Z in which Z is a hydrogenatom, are alkali metal, for example sodium, salts.

In yet another aspect, the present invention provides a method oftreating humans and other animals suffering from microbial infectionswhich comprises administering a non-toxic effective antimicrobialtreatment amount of a compound of formula (II), or preferablyadministering a pharmaceutical formulation comprising the said amount ofa compound of formula (II) and a pharmaceutically acceptable carrier, tothe infected human or other animal.

The compounds of formula (II) may be administered at a dose range of 1to 60 mg/kg bodyweight daily in one or several doses.

Further advantages of the present invention can be ascertained from thefollowing examples which should not be construed as limiting the scopeof the invention in any way.

EXAMPLE 1 6-(1-Methylhydrazino)isocytosine (IV)

A mixture of 6-chloroisocytosine (17.50 g) and methylhydrazine (27.70 g)in water (900 ml) was stirred and refluxed for 3 hours. The resultingsolution was allowed to stand at room temperature for 6 hours then at 0°C. overnight, in order that the product could crystallise out. The whitecrystals were collected by filtration, washed with water (800 ml) andsubsequently with 95% ethanol (200 ml). Drying under vacuum at 70° C.yielded 6-(1-methylhydrazino)isocytosine (11.01 g; 56% of theoreticalyield; m.p. 274°-280° C. decomposition).

Elemental analysis: Calcd. for C₅ H₉ N₅ O.O.5HzO: C,36.58%; H,6.14%;N,42.66%. Found: C,36.42%; H,6.06%; N,42.61%. nmr (DMSO-d₆) δ 3.12(s,3H), 4.47(br s, 2H), 5.00(s, 1H), 6.16(br s, 2H), 9.68(br s, 1H). uv λmax (CH₃ OH) 225.5 nm (ε 24,000), 274(17,300).

EXAMPLE 2 7-Amino-1,3-dimethylpyrimido(4,5-c)pyridazine-4,5-(1H,6H)-dione

    (R.sup.1 ═CH.sub.3)                                    (II)

To a stirred, refluxing solution of 6-(1-methylhydrazino)isocytosinehemihydrate (8.00 g) in water (1L) was added methyl pyruvate (6.00 g).After 70 minutes a greenish-yellow solid was collected by filtration ofthe hot reaction mixture, washed with two portions of water (50 ml each)and dried under vacuum at 70° C. to yield7-amino-1,3-dimethylpyrimido(4,5-c)pyridazine-4,5-(1H, 6H)-dione (5.11g; 51% of theoretical yield; m.p.>300° C.).

Elemental analysis: Calcd. for C₈ H₉ N₅ O₂ : C,46.37%; H,4.38%;N,33.80%. Found: C,46.48%; H,4.42%; N,33.91%. nmr (DMSO-d₆) δ 2.07(s,3H), 3.71(s, 3H), 7.12(br s, 2H), 10.75(br, s, 1H) pK_(a) values4.1±0.1; 8.6±0.1 u.v. λ max (CH₃ OH) 255 nm (ε 40,000), 299.5(7,600),310 sh (5,600).

EXAMPLE 37-Amino-3-acetoxymethyl-1-methylpyrimido(4,5-c)pyridazine-4,5(1L H,6H)-dione

    (R.sup.1 ═CH.sub.3 CO.O.CH.sub.2)                      (II)

To a stirred, refluxing solution of 6-(1-methylhydrazino)isocytosinehemihydrate (0.16 g) in methanol (5 ml) was added methyl3-acetoxy-2-oxo-propanoate (0.19 g). After refluxing for a further 22hours, the solid formed during the course of the reaction was collectedby filtration of the hot reaction mixture and washed with methanol toyield 7-amino-3-acetoxymethyl-1-methylpyrimido(4,5-c)-pyridazine-4,5(1H,6H)-dione (0.11 g; 43% of theoretical yield; m.p.>280° C.).

Elemental analysis: Calcd. for C₁₀ H₁₁ N₅ O₄ : C,45.28%; H,4.18%;N,26.41%. Found: C,45.11%; H,4.24%; N,26.37%. nmr (TFA) δ 2.32(s, 3H),4.27(s, 3H), 5.51 (s, 2H), 7.25(br s, 2H). uv λ max (CH₃ OH) 258 nm (ε37,100), 299.5(7,400).

EXAMPLE 47-Amino-3-hydroxymethyl-1-methylpyrimido(4,5-c)pyridazine-4,5(1H,6H)-dione Sodium Salt

    (R.sup.1 ═CH.sub.2 OH)                                 (II)

To 7-amino-3-acetoxymethyl-1-methylpyrimido(4,5-c)-pyridazine-4,5(1H,6H)-dione (0.100 g) in water (1 ml) was added dropwise with shaking 10%(w/w) aqueous sodium hydroxide (0.25 ml), the orange solution becomingquickly cloudy. The mixture was allowed to stand at room temperature for30 minutes after which time the off-white granular solid which hadformed was collected by filtration, rinsed well with methanol and driedunder vacuum at room temperature to yield7-amino-3-hydroxymethyl-1-methylpyrimido(4,5-c)pyridazine-4,5(1H,6H)-dione as its sodium salt (0.082 g; 81% of theoretical yield;m.p.>300° C.).

Elemental analysis: Calcd. for C₈ H₈ N₅ NaO₃.H₂ O: C,36.50%; H,3.83%;N,26.61%; Na,8.73; Found: C,36.55%; H,3.91%; N,26.50%, Na,8.70. nmr(TFA) δ 4.29(s, 3H), 5.19(s, 2H), 7.20(br s, 2H). uv λ max (0.1 N HCl)255 nm (ε 39,400), 299(7,200).

EXAMPLE 57-Amino-3-(1-ethoxycarbonylethyl)-1-methylpyrimido(4,5-c)-pyridazine-4,5(1H,6H)-dione

    R.sup.1 ═CH(Y)CO.sub.2 Z; Z═C.sub.2 H.sub.5 ; Y═CH.sub.3) (II)

To a stirred, refluxing solution of 6-(1-methylhydrazino)isocytosinehemihydrate (1.86 g) in water (120 ml) was added diethyl3-methyl-2-oxo-succinate (4.59 g). After refluxing for a further 3hours, the solid formed during the course of the reaction was collectedby filtration of the hot reaction mixture, washed with two portions ofwater (20 ml each) and dried under vacuum at 70° C. to yield7-amino-3-(1-ethoxycarbonylethyl)-1-methylpyrimido(4,5-c)pyridazine-4,5(1H,6H)-dione (1.93 g; 58% theoretical yield; m.p.>280° C.).

Elemental analysis: Calcd. for C₁₂ H₁₅ N₅ O₄ : C,49.14%; H,5.16%;N,23.88%. Found: C,49.10%; H,5.18%; N,23.62%. nmr (TFA) δ 1.38 (t, 3H),1.77(d, 3H), 4.28 (s, 3H), 4.41(q, 3H), 7.17(br s, 2H). uv λ max (CH₃OH) 257 nm (ε 41,100), 299.5(7,400), 310 sh (5,600).

EXAMPLE 67-Amino-3-(1-carboxyethyl)-1-methylpyrimido(4,5-c)-pyridazine-4,5(1H,6H)-dione Disodium Salt

    (R.sup.1 ═CH(Y)CO.sub.2 Z; Y═CH.sub.3 ; Z═H)   (II)

A. A mixture of7-amino-3-(1-ethoxycarbonylethyl)-1-methylpyrimido(4,5-c)pyridazine-4,5-(1H,6H)-dione (2.97 g) in 10% (w/w) aqueous sodium hydroxide (67 ml) wasswirled vigorously for 25 minutes. Although a complete solution was notobtained during the agitation, a solid began to precipitate after 20minutes. The mixture was then allowed to stand at room temperature for 1hour before being chilled at 0° C. for 11/2 hours to allow completeprecipitation of the product. The precipitate was collected byfiltration, washed with three portions of 95% ethanol (25 ml each) anddried overnight at room temperature in a vacuum desiccator to yield7-amino-3-(1-carboxyethyl)-1-methylpyrimido(4,5-c)-pyridazine-4,5(1H,6H)-dione disodium salt (2.42 g; 70% of theoretical yield; m.p.>300°;hygroscopic crystals).

Elemental analysis: Calcd. for C₁₀ H₉ N₅ Na₂ O₄.O.5H₂ O: C,37.74%;H,3.17%; N,22.01%; Na,14.45%. Found: C,37.69%; H,3.21%; N,22.05%;Na,14.44%. nmr (TFA) δ 1.81(d, 3H), 4.30(s, 3H), 4.45(q, 1H), 7.17(br s,2H). uv λ max (0.1 N HCl) 255 nm (ε 41,500), 301(7,800).

B. The 3-CH₂ CH₂ CO₂ H compound (disodium salt) was prepared in a mannersimilar to that of 6A from the 3-CH₂ CH₂)CO₂ C₂ H₅ starting materialexcept that collected precipitated solid was washed with CH₃ OH, yield82%. Calcd. for C₁₀ H₉ N₅ Na₂ O₄.O.4CH₃ OH.O.4H₂ O: C,37.94%; H,3.49%;N,21.27%; Na,13.97%. Found: C,37.91%; H,3.21%; N,21.36%; Na,13.99%.

C. The 3-CH(OCH₃)CO₂ H compound (disodium salt) was prepared in a mannersimilar to that of 6A from the 3-CH(OCH₃)CO₂ C₂ H₅ starting material,yield 69%. Calcd. for C₁₀ H₉ N₅ Na₂ O5: C,36.93%; H,2.79%; N,21.54%;Na,14.14%. Found: C,36.70%; H,2.92%; N,21.38%; Na,14.01%.

EXAMPLE 73-Carbomethoxy-5,7-diamino-1-methylpyrimido(4,5-c)-pyridazin-4(1H)-one

    (R.sup.2 ═CH.sub.3)                                    (VI)

To a stirred mixture of 2,4-diamino-6-(1-methylhydrazino)pyrimidine(0.77 g) in anhydrous methanol (50 ml) was added diethyl ketomalonate(1.16 g) at room temperature. An orange solution resulted as the mixturewas heated to reflux over a five minute period. After a further 72 hoursof refluxing, the crude product which had separated out was collected bysuction filtration of the hot mixture, washed with methanol and driedunder reduced pressure at 70° C. to give a pale yellow solid (0.80 g;m.p. 272°-274° C.). Recrystallisation of 0.70 g of this solid frommethanol yielded pure pale yellow3-carbomethoxy-5,7-diamino-1-methylpyrimido(4,5-c)pyridazin-4(1H)-one(0.55 g; m.p. 274°-276° C.).

Elemental analysis: Calcd. for C₉ H₁₀ N₆ O₃ : C,43.20%; H,4.03%;N,33.59%. Found: C,43.12%; H,4.05%; N,33.54%. nmr (DMSO-d₆) δ 3.80 (s,3H), 3.82 (s, 3H), 7.07 (br s, 2H), 7.90(br d, 1H, J═4 Hz), 8.80(br d,1H, J═4 Hz). uv (CH₃ OH) λ max 228 nm (ε 15,200), 255.5(30,300), 261 sh(29,000), 313(8,700).

EXAMPLE 8 7-Amino-3-carboxy-1-methylpyrimido(4,5-c)pyridazine-4,5(1H,6H)-dione Disodium Salt

    (R.sup.1 ═CO.sub.2 H)                                  (II)

A mixture of3-carbomethoxy-5,7-diamino-1-methylpyrimido(4,5-c)pyridazine-4(1H)-one(0.250 g) in 4 N aqueous sodium hydroxide (12.5 ml) was stirred atreflux for 21/2 hours and then allowed to stand at room temperature for1 hour before being filtered. The collected white solid wasrecrystallised twice from water/methanol, dried under vacuum at 70° C.,and allowed to air-equilibrate to give7-amino-3-carboxy-1-methylpyrimido(4,5-c)pyridazine-4,5(1H, 6H)-dione asits disodium salt (0.146 g; 45% of theoretical yield; m.p.>300° C.).

Elemental analysis: Calcd. for C₈ H₅ N₅ O₄ Na₂.2.25H₂ O: C,29.87%;H,2.98%; N,21.77%; Na,14.29%. Found: C,29.60%; H,2.60%; N,21.54%;Na,14.08; nmr (TFA) δ 4.30(s, 3H) 7.12(br s, 2H). uv λ max (pH 2), 266.6nm (ε 45,700), 314.5(6,300).

EXAMPLE 9 5,7-Diamino-1,3-dimethylpyrimido(4,5-c)pyridazine-4(1H)-one

    (R.sup.2 ═CH.sub.3)                                    (VI)

To a refluxing solution of 2,4-diamino-6-(1-methylhydrazino)pyrimidine(500 mg) in anhydrous methanol (15 ml) was added methyl pyruvate (496mg) over a five minute period. Reflux was continued for 5 hours afterwhich time the solid which had separated was collected by suctionfiltration of the hot mixture, washed with methanol, and dried undervacuum at 70° C. to yield tan crystals of5,7-diamino-1,3-dimethylpyrimido(4,5-c)-pyridazine-4-(1H)-one (508 mg;76% of theoretical yield; m.p.>275° C.).

Elemental analysis: Calcd. for C₈ H₁₀ N₆ O: C,46.59%; H,4.89%; N,40.76%.Found: C,46.66%; H,4.98%; N,40.69. nmr (DMSO-d₆) δ 2.14(s, 3H), 3.74(s,3H), 6.84(br s, 2H)*, 7.72(br d, 1H, J=4 Hz)*, 8.96(br d, 1H, J=4 Hz)*.uv λ max (CH₃ OH) 222 nm (ε 12,800), 247(31,100), 306(11,600).

EXAMPLE 10 7-Amino-1,3-dimethylpyrimido(4,5-c)pyridazine-4,5(1H,6H-dione

    (R.sup.1 ═CH.sub.3)                                    (II)

A mixture of 5,7-diamino-1,3-dimethylpyrimido-(4,5-c)pyridazin-4(1H)-one(0.50 g) and 1.5 N aqueous sodium hydroxide (35 ml) was stirred atreflux for 24 hours after which time a small amount of solid was removedby filtration of the hot mixture. On cooling, the yellow filtratedeposited white needles which were collected by filtration and dissolvedin warm water (20 ml). Adjustment of this aqueous solution to pH 5 bydropwise addition of 6 N hydrochloric acid and subsequent cooling toroom temperature provided a very finely divided white precipitate whichwas collected, washed with water and dried under vacuum at 70° C. togive 7-amino-1,3-dimethylpyrimido-(4,5-c)pyridazine-4,5(1H, 6H)-dione(0.38 g; 76% of theoretical yield). The u.v., i.r., and n.m.r. spectraof this compound were identical to those of the sample made according tothe procedure of Example 2.

EXAMPLE 11

Adopting the general procedure of Example 2, that is to say, addition ofthe appropriate α-ketoester of formula (V) to a refluxing mixture orsolution prepared from a very pure, appropriately substitutedalkylhydrazinoisocytosine of formula (IV) and filtered solvent in theproportion of 1 g in 100 ml, collected by filtration of the precipitatedcompound of formula (II) from the hot reaction mixture, washing with asmall portion of fresh reaction solvent and drying under vacuum at 70°C., the following compounds of formula (II) were prepared:

    __________________________________________________________________________                     Molar Ratio                                                                           Reflux Solvent    ELEMENTAL ANALYSIS                 R.sup.1          (V:IV)  & Reflux time                                                                           Yield (%)                                                                             F = FOUND; Ca                      __________________________________________________________________________                                               = CALCULATED                       CH(Y)CO.sub.2 Z  1.7:1   CH.sub.3 OH                                                                             37      Ca: C47.31% H4.69% N25.08%         Y = H, Z = C.sub.2 H.sub.5                                                                             48 hours           F: C47.40% H4.78% N25.01%         CH.sub.2 C.sub.6 H.sub.5                                                                       1.1:1   CH.sub.3 OH                                                                             53      Ca: C59.35% H4.63% N24.72%                                  (under N.sub.2)    F: C59.33% H4.65% N24.65%                                  48 hours                                             CH.sub.2 C.sub.6 H.sub.4 (NO.sub.2)(2)                                                         1.5:1   CH.sub.3 OH                                                                             40      Ca: C51.22% H3.68% N25.60%                                  26 hours           F: C51.20% H3.71% N25.58%         CH.sub.2 C.sub.6 H.sub.3 (OCH.sub.3).sub.2 (3,4)                                               1.5:1   CH.sub.3 OH                                                                             38      Ca: C55.97% H4.99% N20.40%                                  (under N.sub.2)    F: C56.07% H5.06% N20.27%                                  42 hours                                             C.sub.6 H.sub.5  1.5:1   1:1 C.sub.2 H.sub.5 OH/H.sub.2 O                                                        56      Ca: C57.99% H4.12% N26.01%                                  27 hours           F: C57.96% H4.17% N25.98%         CH.sub.2 CH.sub.2 CO.sub.2 C.sub.2 H.sub.5                                                     1.5:1   H.sub.2 O 60      Ca: C49.14% H5.16% N23.88%                                  2 hours            F: C49.05% H5.22% N24.07%         C.sub.2 H.sub.5  1.5:1   CH.sub.3 OH                                                                             82      Ca: C48.86% H5.01% N31.66%                                  72 hours           F: C48.96% H5.00% N31.45%         n-C.sub.3 H.sub.7                                                                              1.5:1   H.sub.2 O 51      Ca: C51.05% H5.57% N29.77%                                  140 minutes        F: C51.13% H5.61% N29.67%         n-C.sub.6 H.sub.13                                                                             1.5:1   CH.sub.3 OH                                                                             67      Ca: C56.30% H6.91% N25.26%                                  48 hours           F: C56.28% H6.95% N25.17%         iso-C.sub.4 H.sub.9                                                                            1.5:1   CH.sub.3 OH                                                                             62      Ca: C53.00% H6.07% N28.10%                                  48 hours           F: C53.05% H6.12% N27.97%         3-Indolylmethyl  1.2:1   6:1 CH.sub.3 OH/H.sub.2 O                                                               11      Ca: C59.62% H4.38% N26.08%                                  21 hours           F: C59.53% H4.41% N26.12%         3-Indolyl        1.5:1   CH.sub.3 OH                                                                             36      Ca: C58.43% H3.92% N27.26%                                  5 days             F: C58.47% H3.95% N27.14%         CH(OCH.sub.3)CO.sub.2 C.sub.2 H.sub.5                                                          1.5:1   CH.sub.3 OH                                                                             39      Ca: C46.60% H4.89% N22.65%                                  9 days             F: C46.62% H4.93% N22.63%          ##STR8##        1.2:1   CH.sub.3 OH (under N.sub.2) 50                                                          25urs   Ca: C54.68% H5.13% N18.76%  F:                                                C54.71% H5.14% N18.81%              ##STR9##        1.2:1   CH.sub.3 OH 166 hours                                                                   34 (after recrystall- sation from                                             CH.sub.3 OH)                                                                          Ca: C55.33% H4.89% N20.38%  F:                                                C55.11% H4.87% N20.30%             CH(Y)CO.sub.2 Z  1.7:1   CH.sub.3 OH                                                                             37      Ca: C47.31% H4.69% N25.08%         Y = H, Z = C.sub.2 H.sub.5                                                                             48 hours           F: C47.40% H4.78% N25.01%         CH.sub.2 C.sub.6 H.sub.5                                                                       1.1:1   CH.sub.3 OH                                                                             53      Ca: C59.35% H4.63% N24.72%                                  (under N.sub.2)    F: C59.33% H4.65% N24.65%                                  42 hours                                             CH.sub.2 C.sub.6 H.sub.4 (NO.sub.2)(2)                                                         1.5:1   CH.sub.3 OH                                                                             40      Ca: C51.22% H3.68% N25.60%                                  26 hours           F: C51.20% H3.71% N25.58%         CH.sub.2 C.sub.6 H.sub.3 (OCH.sub.3).sub.2                                                     1.5:1   CH.sub.3 OH                                                                             38      Ca: C55.97% H4.99% N20.40%                                  (under N.sub.2)    F: C56.07% H5.06%  N20.27%                                 42 hours                                             __________________________________________________________________________

EXAMPLE 12 7-Amino-3-phenacyl-1-methylpyrimido(4,5-c)pyridazine-4,5-(1H,6H)-dione

    (R.sup.1 ═CH.sub.2 CO.C.sub.6 H.sub.5)                 (II)

To a stirred, refluxing mixture of 6-(1-methylhydrazino)isocytosinehemihydrate (1.00 g) in methanol (100 ml) was added ethylbenzoylpyruvate (2.01 g). After 67 hours yellowish-brown solid wascollected from the hot reaction mixture, washed with three portions ofmethanol totalling 20 ml, and dried under vacuum at 75° C., yield 0.130g (7%): m.p.>300°; nmr (CF₃ COOH) δ 4.28 (s, 3H), 4.87(s, 2H), 7.17(brs, 2H), 7.4-8.3(m, 5H); uv λ max (CH₃ OH) 259 nm (ε 44,900), 301(8,300),310 sh (6,900), 375 sh (900). Mass spectrum (240°): M, m/e 311, 17%; m/e166, 1%; m/e 105, 100%. The following accurate mass was determined:166.0487 (C₆ H₆ N₄ O₂).

Anal. Calcd. for C₁₅ H₁₃ N₅ O₃ : C,57.87%; H,4.21%; N,22.50%. Found:C,57.80%; H,4.26%; N,22.46%.

EXAMPLE 137-Amino-3-(3-hydroxyphenacyl)-1-methylpyrimido(4,5-c)-pyridazine-4,5(1H,6H)-dione

    (R.sup.1 ═CH.sub.2 CO.C.sub.6 H.sub.4 OH)              (II)

Adopting the general procedure of Example 12, the above compound wassynthesised and isolated.

Reaction time of 22 hours. Yield 7%: m.p. 290°-295° dec; nmr (CF₃ COOH)δ 4.28 (s, 3H), 4.83 (s, 2H), 7.16(br s, 2H), 7.4-8.0(m, 4H); uv λ max(CH₃ OH) 213.5 nm (ε 26.300), 259(47,400), 303(10,600), 309 sh (9,700).

Anal. Calcd. for C₁₅ H₁₃ N₅ O₄.O.5H₂ O: C,43.16%; H,5.55%; N,16.78%.Found: C,43.15%; H,5.59%; N,16.83%.

EXAMPLE 147-Amino-3-(2,4,6-trimethoxyphenacyl)-1-methylpyrimido-(4,5-c)pyridazine-4,5(1H,6H)-dione

    (R.sup.1 ═CH.sub.2 CO.C.sub.6 H.sub.2 (OCH.sub.3).sub.3) (II)

Adopting the general procedures of Example 12, the above compound wassynthesised and isolated.

Reaction time of 191/2 hours. Yield 5%: m.p. 280° dec; nmr (CF₃ COOH) δ4.18, 4.24 and 4.25 (overlapping s's, 12H), 4.96(s, 2H), 6.52(s, 2H),7.22(br s, 2H); uv λ max (CH₃ OH) 258 nm (ε 37,500), 296.5 sh (12,700),311.5 sh (9,800).

Anal. Calcd. for C₁₈ H₁₉ N₅ O₆ : C,53.86%; H,4.77%; N,17.45%. Found:C,53.68%; H,4.81%; N,17.46%.

EXAMPLE 157-Amino-3-(2,5-dimethoxyphenacyl)-1-methylpyrimido-(4,5-c)pyridazine-4,5(1H,6H)-dione

    (R.sup.1 ═CH.sub.2 CO.C.sub.6 H.sub.3 (OCH.sub.3).sub.2) (II)

To a stirred, refluxing mixture of 6-(1-methylhydrazino)isocytosinehemihydrate (4.00 g) in methanol (400 ml) was added methyl2,5-dimethoxybenzoylpyruvate (7.14 g). After 19 hours reddish-orangesolid was collected from the hot mixture, washed with two portions ofmethanol totalling 50 ml, and dried under vacuum at 75° to yield 0.628g. This solid was an inseparable 1:1 mixture of the desired 4,5-dioneand its 3,5-dione isomer.

The filtrate was refluxed an additional 22.5 hours, and pale yellowsolid was collected from the hot mixture, washed with several portionsof methanol totalling 30 ml, and dried under vacuum at 75°, yield 0.09 g(1%): m.p.>300°; nmr (CF₃ COOH) δ 4.02(s, 3H), 4.07(s, 3H), 4.28(s, 3H),4.90(s, 2H), 6.8-7.7(m, 5H); uv λ max (CH₃ OH) 223 nm weak sh (ε22,800), 258.5(48,500), 302.5(10,000), 311.5 sh (9,000), 332.5 sh(5,500).

Anal. Calcd. for C₁₇ H₁₇ N₅ O₅ : C,54.98%; H,4.61%; N,18.86%. Found:C,54.68%; H,4.64%; N,19.03%.

EXAMPLE 167-Amino-3-(2,4-dimethoxyphenacyl)-1-methylpyrimido-(4,5-c)pyridazine-4,5(1H,6H)-dione

    (R.sup.1 ═CH.sub.2 CO.C.sub.6 H.sub.3 (OCH.sub.3).sub.2 (II)

Following the general procedure of Example 15, the above compound wassynthesised and isolated.

A 2:1 mixture of 4,5-dione and 3,5-dione isomers, respectively, wascollected after 18 hours. The filtrate was refluxed an additional 47hours for a 9% yield of 4,5-dione isomer: m.p. 290°-300° dec; nmr (CF₃COOH) δ 4.02 and 4.06 overlapping s's, (6H), 4.27(s, 3H), 4.84(s, 2H),6.6-8.2(m, 5H); uv λ max (CH₃ OH) 227.5 nm (ε 20,200), 259.5(40,700),304(17,400), 413(2,800), 435(2,700), 460 (2,900).

Anal. Calcd. for C₁₇ H₁₇ N₅ O₅ : C,54.98%; H,4.61%; N,18.86%. Found:C,54.97%; H,4.69%; N,18.98%.

EXAMPLE 177-Amino-3-(3,4-dimethoxyphenacyl)-1-methylpyrimido-(4,5-c)pyridazine-4,5(1H,6H)-dione

    R.sup.1 ═CH.sub.2 CO.C.sub.6 H.sub.3 (OCH.sub.3).sub.2 (II)

Following the general procedure of Example 15, the above compound wassynthesised and isolated.

An insoluble mixture was collected after 17 hours. The filtrate wasrefluxed an additional 47 hours for a 2% yield of 4,5-dione isomer:m.p.>300°; nmr (CF₃ COOH) δ 4.04 and 4.08 (overlapping s's, 6H), B4.28(s, 3H), 4.83 (s, 2H), 7.0-7.4(m, 3H), 7.7-8.2(m, 2H); uv λ max (CH₃OH) 229 nm (ε 23,300), 259(42.000), 274 sh (22,200), 304 (18.700).

Anal. Calcd. for C₁₇ H₁₇ N₅ O₅.0.1H₂ O: C,54.72%; H,4.65%; N,18.77%.Found: C,54.71%; H,4.68%; N,18.71%.

EXAMPLE 187-Amino-3-(3,4,5-trimethoxyphenacyl)-1-methylpyrimido-(4,5-c)pyridazine-4,5(1H,6H)-dione

    (R.sup.1 ═CH.sub.2 CO.C.sub.6 H.sub.2 (OCH.sub.3).sub.3 (II)

Following the general procedure of Example 15, the above compound wassynthesised and isolated.

A 1:1 mixture of 4,5-dione and 3,5-dione isomers, respectively, wascollected after 181/2 hours. The filtrate was refluxed an additional 23hours for a 2% yield of 4,5-dione isomer: m.p.>300°; nmr (CF₃ COOH) δ4.07 and 4.13 (overlapping s's, 9H), 4.30(s, 3H), 4.86(s, 2H), 7.18(brs, 2H), 7.54(s, 2H); uv λ max (CH₃ OH) 213 nm (ε 32,500), 258.5(43.700),297 sh (17,200), 310 sh (13,700). Mass spectrum (250°): M, m/e 401, 7%;m/e 195, 100%; m/e 166, 2%. The following accurate mass was determined:166.0488 (C₆ H₆ N₄ O₂).

Anal. Calcd. for C₁₈ H₁₉ N₅ O₆ : C,53.86%; H,4.77%; N,17.45%. Found:C,53.82%; H,4.85%; N,17.55%.

EXAMPLE 19

Potential inhibitors of DHPB synthesis may be tested by investigatingthe inhibitory effect they impose in the enzymes responsible for thebiosynthesis of dihydropteroic acid (DPtA), namelyhydroxymethyldihydropteridine pyrophosphokinase (HMPPS), anddihydropteroate synthetase, hereinafter referred to as `synthetase`. Inthe following reaction equations the compounds are referred to by theirabbreviated forms defined hereinbefore in the specification.

1. HMPPS:

    HMPt+ATP .sup.M g.spsp.2+ Pt+AMP

2. `Synthetase`:

    Pt+pAB .sup.M g.spsp.2+ DPtA+pyrophosphate

This reaction requires two enzymes since the starting substrates are H₂ptCH₂ OH, ATP, and PAB, and the products are H₂ pteroate and AMP. Incrude extracts of E. coli (and the 0-50% ammonium sulfate fraction usedby us) the first enzyme,2-amino-4-hydroxy-6-hydroxymethyl-7,8-dihydropteridine pyrophosphokinase("kinase"), has a threefold lower specific activity than the secondenzyme, dihydropteroate synthetase ("synthetase").

The reactions are followed by determining the amount of ¹⁴ C in H₂pteroate after separation from the substrate, p-aminobenzoate-7-¹⁴ C, bypaper chromatography.

The following results were obtained by the coupled assay method.

    ______________________________________                                                            Concentration in μM                                                        required to give 50%                                      Compound of formula R.sup.1                                                                       inhibition of DHPB                                        ______________________________________                                        CH.sub.3            16.0                                                      C.sub.6 H.sub.5     15.0                                                      CH.sub.2 OH         12.0                                                      CH.sub.2 OCOCH.sub.3                                                                              15.0                                                      CO.sub.2 H (disodium salt)                                                                        21.0                                                      CH(CH.sub.3)CO.sub.2 C.sub.2 H.sub.5                                                              1.6                                                       CH(CH.sub.3)CO.sub.2 H                                                                            3.7                                                       CH.sub.2 CH.sub.2 CO.sub.2 H                                                                      5.5                                                       CH.sub.2 CH.sub.2 CO.sub.2 CH.sub.2 H.sub.5                                                       16.0                                                      C.sub.2 H.sub.5     33.0                                                      CH.sub.2 C.sub.6 H.sub.5                                                                          11.0                                                      n-C.sub.3 H.sub.7   45.0                                                      n-C.sub.6 H.sub.13  50.0                                                      iso-C.sub.4 H.sub.9 20.0                                                       ##STR10##          12.0                                                      3-Indolylmethyl     3.0                                                       3-Indolyl           30.0                                                      CH.sub.2 CO.sub.2 C.sub.2 H.sub.5                                                                 6.2                                                       CH(OCH.sub.3)CO.sub.2 C.sub.2 H.sub.5                                                             17.0                                                      CH(OCH.sub.3)CO.sub.2 H                                                                           2.8                                                       CH.sub.2 COC.sub.6 H.sub.5                                                                        0.76                                                       ##STR11##          0.15                                                       ##STR12##          0.03                                                       ##STR13##          0.14                                                       ##STR14##          0.77                                                       ##STR15##          0.06                                                       ##STR16##          2.7                                                        ##STR17##          0.22                                                       ##STR18##          0.86                                                       ##STR19##          11.0                                                      ______________________________________                                    

What is claimed is:
 1. A compound of the formula (II): ##STR20## or atautomer, or a pharmaceutically acceptable salt thereof, wherein R¹is:lower alkyl; hydroxymethyl; phenyl; carboxy; benzyl; benzylsubstituted in the phenyl ring with nitro or lower alkoxy; phenacyl;phenacyl substituted in the phenyl ring with hydroxy or lower alkoxy; acarboxylic acid acyloxymethyl of from 1 to 6 carbon atoms arranged in astraight or branched chain;indolyl; indolyl methyl; CH(CN)CH₂ C₆ H₅ ;CH(CN)CH₂ C₆ H₅ substituted in the phenyl ring with lower alkoxy; orCH(Y)CO₂ Z or CH₂ CH₂ CO₂ Z in which Y is hydrogen or lower alkyl oralkoxy and Z is hydrogen or lower alkyl.
 2. A compound according toclaim 1 wherein R¹ is:methyl; phenyl; benzyl; benzyl substituted in thephenyl ring with nitro or 2 or 3 methoxys; hydroxymethyl; phenacyl;phenacyl substituted in the phenyl ring with hydroxy, or 2 or 3methoxys; CH₂ CH₂ CO₂ H; acetoxymethyl; indolyl; CH(CN)CH₂ C₆ H₅substituted in the phenyl ring with 3 methoxys; or CH₂ (Y)CO₂ Z in whichY is methyl, methoxy, or hydrogen and Z is hydrogen or C₁₋₄ alkyl.
 3. Acompound according to claim 1 wherein R¹ is:benzyl; phenacyl; orphenacyl substituted in the phenyl ring with hydroxy or 2 or 3 methoxys.4. A compound according to claim 2 wherein R¹ is:benzyl; phenacyl; orphenacyl substituted in the phenyl ring with hydroxy or 2 or 3 methoxys.5. A compound selected from the group consisting of7-amino-1,3-dimethyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)-pyridazine;7-amino-1-methyl-3-phenyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-hydroxymethyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-benzyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)-pyridazine;7-amino-1-methyl-3-(2-nitrobenzyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-indolylmethyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)-pyridazine;7-amino-1-methyl-3-acetoxymethyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-(1-carboxyethyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-(2-carboxymethyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-(ethoxycarbonylmethyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-((1-methoxy)-carboxymethyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-(α-cyano-3,4,5-trimethoxyphenethyl)-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine;7-amino-1-methyl-3-(1-ethoxycarbonylethyl)-4-oxo-5-hydroxy-,4-dihydropyrimido(4,5-c)pyridazineand tautomers and pharmaceutically acceptable salts thereof. 6.7-amino-3-benzoylmethyl-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazine or a tautomer or a pharmaceutically acceptable salt thereof.7.7-amino-3-(3,4-dimethoxybenzoyl)methyl-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazineor a tautomer or a pharmaceutically acceptable salt thereof. 8.7-amino-3-(3,4,5-trimethoxybenzoyl)methyl-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazineor a tautomer or a pharmaceutically acceptable salt thereof. 9.7-amino-3-(2,4-dimethoxybenzoyl)methyl-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazineor a tautomer or a pharmaceutically acceptable salt thereof. 10.7-amino-3-(3,4,5-trimethoxybenzyl)-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazineor a tautomer or a pharmaceutically acceptable salt thereof. 11.7-amino-3-(2,5-dimethoxybenzoyl)methyl-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridiazineor a tautomer or a pharmaceutically acceptable salt thereof. 12.7-amino-3-(2,4,6-trimethoxybenzoyl)methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazineor a tautomer or a pharmaceutically acceptable salt thereof. 13.7-amino-3-(3-hydroxybenzoyl)-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)-pyridazineor a tautomer or a pharmaceutically acceptable salt thereof. 14.7-amino-3-(3,4-dimethoxybenzyl)-1-methyl-4-oxo-5-hydroxy-1,4-dihydropyrimido(4,5-c)pyridazineor a tautomer or a pharmaceutically acceptable salt thereof.